C++与Java之间的Binder通信

简介

在Android系统开发中经常会碰到server端和client语言不同问题，例如使用C++编写的Service，客户端是Java/Kotlin；或者是app中创建的Service，client端是c++的情况，本篇文章介绍使用C/C++编写的程序如何与Java编写的Service进行binder通信。

1. Binder通信首先创建AIDL文件，用于定义服务端的接口，这里简单示例：

// server
package com.lu.test;
import com.lu.test.ITestClient;

interface ITestService{
    String getServiceName();
    void registerClient(ITestClient client);
}

//client
package com.lu.test;

interface ITestClient{
    String getClientName();
}

2. 编写脚本用于生成c层的头文件(java可以通过Android Studio 生成相关的类)

  cc_library_shared {
    name: "lib-test",
    srcs:["./**/*.aidl"],
    aidl:{
        include_dirs:["./"],
    },
    shared_libs:[
        "libutils",
        "libcutils",
        "libbinder",
    ],
}

我们当前的目录结构如下：

├── Android.bp
└── com
    └── lu
        └── test
            ├── ITestClient.aidl
            └── ITestService.aidl

将该部分文件放入到aosp下，可以放在vendor底下，然后运行在根目录运行:

make lib-test

即可得到头文件：

# 生成的头文件路径
# out/soong/.intermediates/vendor/test/lib-test/android_arm64_armv8-a_shared/gen

生成的文件列表

.
└── com
    └── lu
        └── test
            ├── BnTestClient.h
            ├── BnTestService.h
            ├── BpTestClient.h
            ├── BpTestService.h
            ├── ITestClient.cpp
            ├── ITestClient.cpp.d
            ├── ITestClient.h
            ├── ITestService.cpp
            └── ITestService.h

其中Bn开头的是作为Binder中的server端的头文件，需要我们去实现；Bp打头的文件类似于Java中的Stub，用于做类型转换的代理类。

3. 实现Bn，在这个例子中，我们是需要双向通信的，client端获取ItestService访问server端，同时像server端注册ITestClient，server端通过ITestClient可以访问client端;

• Server端通过Java实现ITestService，使用AS创建一个App，并且编写一个Service即可：

private const val TAG = "TestService"

class TestService : Service() {

    private val mService = object : ITestService.Stub() {
        override fun getServiceName(): String {
            return "TestService";
        }

        override fun registerClient(client: ITestClient) {
            Log.d(TAG, "registerClient : ${client.clientName}")
        }
    }

    override fun onBind(intent: Intent?): IBinder? {
        return mService
    }

    override fun onCreate() {
        super.onCreate()
        //将service添加到ServiceManager管理中
        ServiceManager.addService("BinderTest", mService)
    }
}

• Client端通过C++实现ITestClient
  首先我们看下通过AIDL生成的ITestClient.h

#pragma once

#include <binder/IBinder.h>
#include <binder/IInterface.h>
#include <binder/Status.h>
#include <utils/String16.h>
#include <utils/StrongPointer.h>

namespace com {

namespace lu {

namespace test {

class ITestClient : public ::android::IInterface {
public:
  DECLARE_META_INTERFACE(TestClient)
  virtual ::android::binder::Status getClientName(::android::String16* _aidl_return) = 0;
};  // class ITestClient

class ITestClientDefault : public ITestClient {
public:
  ::android::IBinder* onAsBinder() override {
    return nullptr;
  }
  ::android::binder::Status getClientName(::android::String16*) override {
    return ::android::binder::Status::fromStatusT(::android::UNKNOWN_TRANSACTION);
  }
};  // class ITestClientDefault

}  // namespace test

}  // namespace lu

}  // namespace com

创建一个文件TestClient.h

#ifndef BINDERTEST_TESTCLIENT_H
#define BINDERTEST_TESTCLIENT_H

#include "com/lu/test/BnTestClient.h"

//此处继承的是BnTestClient，这个类帮助我们实现了binder接口的转化
class TestClient : public ::com::lu::test::BnTestClient {
public:
    TestClient();

    virtual ~TestClient();

    ::android::binder::Status getClientName(::android::String16 *_aidl_return);
};

#endif //BINDERTEST_TESTCLIENT_H

创建TestClient.cpp

#include "TestClient.h"

using namespace com::lu::test;

TestClient::TestClient() = default;

TestClient::~TestClient() = default;

::android::binder::Status TestClient::getClientName(::android::String16* _aidl_return){
    *_aidl_return = android::String16("TestClient");
   return android::binder::Status::ok();
}

4. 编写client端的测试程序TestMain.cpp：

#include <unistd.h>
#include "binder/IBinder.h"
#include "utils/StrongPointer.h"
#include "binder/IServiceManager.h"
#include <android/binder_manager.h>
#include <android/binder_process.h>
#include "com/lu/test/ITestService.h"
#include "android_log_define.h"
#include "TestClient.h"
#include "thread"

#define SERVER_NAME  "BinderTest"

using namespace std;
using namespace android;

TestClient *clientImpl = new TestClient();;

android::sp<com::lu::test::ITestService> getService() {
    sp<IServiceManager> sm = defaultServiceManager();
    if (sm == nullptr) {
        LOGE("can't get serviceManager");
        return nullptr;
    }
    auto binder = sm->getService(String16(SERVER_NAME));
    if (binder == nullptr) {
        LOGE("can not get binder");
        return nullptr;
    }

    auto logServer = interface_cast<com::lu::test::ITestService>(binder);
    if (logServer == nullptr) {
        LOGE("can't cast LogServer");
        return nullptr;
    }

    return logServer;
}

int main() {
    auto service = getService();
    if (service == nullptr) {
        LOGE("registerService failed service is null");
        return -1;
    }
    service->registerClient(clientImpl);
    auto name = new String16();
    service->getServiceName(name);
    LOGD("the service name is %s", name->string());
    //这2句是使当前线程具有binder的能力，会阻塞住当前线程，建议可以放到子线程中
    ABinderProcess_setThreadPoolMaxThreadCount(0);
    ABinderProcess_joinThreadPool();
}

编译脚本

cc_binary {
    name: "BindClientTest",
    srcs:[
        "./**/*.cpp"
    ],
    local_include_dirs:[
        "./include",
    ],
    shared_libs:[
        "libutils",
        "libcutils",
        "libbinder",
        "liblog",
        "libbase",
        "libbinder_ndk"
    ],
    cflags: [
        "-Wall",
        "-Werror",
        "-Wextra",
        "-Wno-unused-parameter",
        "-std=c++11",
        "-frtti",
        "-fexceptions",
        "-fPIC",
    ],
}

目录结构

├── Android.bp
├── TestClient.cpp
├── TestMain.cpp
└── include
    ├── TestClient.h
    ├── android_log_define.h
    └── com
        └── lu
            └── test
                ├── BnTestClient.h
                ├── BnTestService.h
                ├── BpTestClient.h
                ├── BpTestService.h
                ├── ITestClient.cpp
                ├── ITestClient.cpp.d
                ├── ITestClient.h
                ├── ITestService.cpp
                └── ITestService.h

5. 调试

• 将Server端的app运行起来
• 将client端放入aosp环境编译，产物推到system/bin/目录下并运行